The invention relates to a device for charging accumulators (batteries in American idiom) comprising means for charging an accumulator by a charging current in separate, sequential time intervals.
Accumulators involve the problem that in the event of overcharging, that is to say, charging with an excessively high current with an excessively high voltage or for an excessively long time water is separated out in the gaseous form. This problem occurs with an accumulator comprising a liquid electrolyte, for example, the lead-sulphuric acid accumulator generally used in motorcars, the so-called "wet" accumulator as well as in the accumulator comprising a dry electrolyte, the so-called "dry" accumulator.
The decomposition of water into gas in the event of overcharging of the accumulator results in both kinds of accumulators in that the capacity of the accumulator is reduced. With the accumulator having a liquid electrolyte this problem is less serious because the drop in capacity can be compensated for by adding distilled water. With accumulators having a dry electrolyte, however, this problem is onerous because the gasification of water from the accumulator, reducing the capacity, cannot be compensated for by replenishing water or by other measures.
In order to solve this problem it has been suggested for accumulators having a dry electrolyte to charge them with a constant current. An advantage of such a charging system is said to be that, when the accumulator is fully discharged, it can be simply calculated when the charging device has to be switched off; however, with an accumulator in need of charging it is only rarely known to what extent it is discharged.
A disadvantage of this known system is that in an attempt to charge the accumulator having a dry electrolyte within reasonable time by using a comparatively high charging current, overcharging will occur towards the end of the charging operation so that the water decomposes more rapidly. An even greater disadvantage of this system with constant charging current resides in that a high degree of overcharging occurs when the charging device is not switched off in due time.
Even when the accumulator having a dry electrolyte is charged by means of a low charging current, overcharging may nevertheless occur, when the charging current is not switched off in due time, apart from the disadvantage of a very long charging time.
There has furthermore been suggested a device for charging accumulators providing a constant voltage, which device may be constructed in different ways. The advantage of this known device is that the charging current decreases according as the accumulator charge increases so that the risk of overcharging is reduced. Nevertheless failing to switching off the accumulator charging device in due time results also in overcharging and gas development. A further disadvantage resides in that the curve of the charging current and hence the duration of the charging operation can be influenced only with difficulty and overcharging is unavoidable unless very long charging periods are allowed and the charging current is interrupted in due time.
U.S. Pat. No. 3,854,082 discloses an accumulator charging device comprising means for alternately charging the accumulator with a constant charging current for a given first period, during which the terminal voltage of the accumulator rises, and with a decreasing charging current for a subsequent period, during which the terminal voltage remains substantially constant.
A disadvantage thereof is that the initial current intensity, which is in this case the maximum current intensity, is maintained until the desired final voltage of the accumulator is attained, which will certainly result in the development of gas.
A further disadvantage is that the accumulator voltage has to be maintained at the constant high final value for a comparatively long time in order to fully charge the accumulator. It is generally known that gas is always evolved from accumulators when it is held at the final voltage.
A further disadvantage is that only the initial current intensity and the final voltage can be adjusted so that optimum matching of the type of accumulator is out of the question so that with the device disclosed in said patent development of gas cannot be avoided.
Moreover, the circuit arrangement for charging accumulators disclosed in said U.S. Pat. No. 3,854,082 includes, for regulating the charging current, a thyristor element, which has a negative influence on the lifetime of the accumulator, as is known to the average expert in this domain.